Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
900 PM EDT Tue Apr 29 2025

Day 1
Valid 01Z Wed Apr 30 2025 - 12Z Wed Apr 30 2025

...THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE
SOUTHERN PLAINS...


...Southern Plains to the Mid Mississippi Valley...
01Z Update...

Continued the southern adjustment, especially on the northern edge,
of the outlook areas extending from western Texas northeastward
across Oklahoma. Recent runs of the HRRR, as well as the 18Z HREF,
indicate multiple convective rounds training northeastward across
the region, producing several inches of rain, with numerous flash
floods likely across portions of the region. The latest guidance
shows the greatest threat extending from Northwest Texas across the
Wichita Falls area into southwestern and south-central Oklahoma --
brushing the southern extent of the OKC metro. Deterministic runs
of the HRRR show accumulations of 7+ inches, while the HREF shows
high neighborhood probabilities for accumulations of 5 inches or
more through the overnight across this region. The Slight and
Moderate Risk areas were shifted a bit farther south across the
southern Texas Panhandle and western Oklahoma, where limited
instability in the most recent analysis supports guidance
indicating a lesser threat for heavy rainfall rates and flash
flooding.

Further to the east, pulled the Marginal and the Slight Risk areas
back west of the Mississippi. Relatively drier and more stable air
is expected to persist until showers and storms now developing
back to the southwest spread into the region late in the period,
with the general model consensus showing relatively lighter amounts
and a lesser flooding threat.

...Ohio Valley to the Central Appalachians...
01Z Update...

In addition to brief heavy downpours across sensitive areas, some
training along the southwest flank of the storms currently moving
across the region may raise the threat for locally heavy rains and
flash flooding. But overall, storms are expected to move
progressively across the area, limiting the potential for
widespread heavy rainfall and flash flooding concerns.

Pereira

Day 2
Valid 12Z Wed Apr 30 2025 - 12Z Thu May 01 2025

...THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
SOUTH-CENTRAL AND SOUTHEAST OKLAHOMA, NORTH TEXAS, AND WESTERN
ARKANSAS...

The primary change for this updated ERO for Wednesday was to
recenter the Moderate Risk further south (approx. 50 mi). This was
due to a corresponding shift in guidance, especially hi-res
guidance, but also the UKMET, CMC, and ECMWF AIFS. It also fits
with the updated expectations for the current Day 1 period, as
guidance has signaled the potential for relatively strong cold
pools reinforcing the effective front and gradually pushing it
further south. This would center it more squarely near the Red
River (OK-TX border region) or perhaps fully into N TX. There is
still some uncertainty associated with the exact convective
evolution over the 12-18 hours immediately prior to the start of
the Day 2 period, so the Moderate Risk and Slight Risk were kept
fairly broad to account for different scenarios. Nevertheless,
there was a clear enough shift in the guidance to warrant an
adjustment in the overall risk contours.

A significant flash flooding event could be ongoing at the start of
the period (~12Z Wednesday) over western North Texas and/or south-
central and southwest Oklahoma. Late tonight the southward drift of
the effective front and the associated zone of active convection
may begin to slow or stall as the low-level jet (LLJ) begins to
ramp up considerably over Texas. Where the nose of the LLJ sets up
relative to ongoing convection will go a long way to determining
the primary area of concern for Wednesday. Most hi-res models show
the convective cluster(s) propagating east through the morning
toward AR and N LA, with renewed development upstream over N TX by
the afternoon and another training and backbuilding configuration.

This same guidance generally supports the idea of a 24-hour
rainfall maximum in excess of 7 inches somewhere in the new
Moderate Risk area, but there is lingering uncertainty as to
precisely where that will occur. Although the probability is
currently highest in the vicinity of the Red River near where SE OK
borders N TX, it's conceivable it could occur closer to or inside
the DFW metro area. The metro area is now fully in the Moderate
Risk area with this latest update.

Some significant flash flooding will be possible as much of the
rain will likely fall in 6-12 hours and the high end amounts
produced by some of the guidance would approach or exceed a 100
year ARI for those durations. Neighborhood probabilities of
100 year ARI exceedance are not particularly high, reflecting both
the uncertainty of the exact placement of the heaviest amounts, and
that amounts well in excess of those thresholds are not currently
supported by guidance. Significant and life-threatening flash
flooding would be most likely in two scenarios:

(1) In western North Texas or south-central Oklahoma at the
beginning of the period, as a continuation from the previous night.
These areas have been quite wet over the past week or so, so there
is existing vulnerability to extreme rainfall. (This is more
likely.)

(2) In the Dallas-Ft. Worth metro area, as the rainfall in question
would cause serious issues in a highly urbanized area. (This is
also possible, but not certain, and highly dependent on the exact
convective evolution.)

Further north, a broad Marginal and Slight Risk area was maintained
for the Mid Mississippi Valley and Central Plains. There is some
signal for organized convection and localized heavy rainfall,
including a scattering of low probabilities of FFG exceedance, but
confidence in any specific scenario is lacking.

Lamers

Day 3
Valid 12Z Thu May 01 2025 - 12Z Fri May 02 2025

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL OVER A BROAD AREA
FROM THE GREAT LAKES TO MID MISSISSIPPI VALLEY TO WESTERN GULF
COAST...

An expansive plume of instability is expected to support scattered
to numerous areas of thunderstorms during the day 3 period ahead of
a surface low and trailing cold front. Machine learning
probabilities from Colorado State support the idea of a broad
Marginal Risk; this is what was inherited and it was generally
maintained. The biggest change was to extend the area southwest
through portions of Louisiana and south and southeast Texas.
Confidence in specifics is low, and the overall flash flood threat
should be limited by the fairly progressive nature of the
convection. Precipitable water values are more anomalous in the
Great Lakes region (above the 90th percentile; above 1.3 inches),
but that is also where thunderstorms should be moving the fastest.
Meanwhile, available moisture will be less anomalous further south,
in TX and LA, but thunderstorms will be moving slower. Therefore,
heavy rainfall ingredients will generally be offset across the
whole region. Hi-res models that extend out beyond 48 hours show
more significant localized rainfall maxima in TX and LA, which
makes sense given stronger instability and slower overall storm
motions. This may be where a targeted upgrade to a Slight Risk is
eventually needed.

Lamers


Day 1 threat area:
https://www.wpc.ncep.noaa.gov/qpf/94epoints.txt
Day 2 threat area:
https://www.wpc.ncep.noaa.gov/qpf/98epoints.txt
Day 3 threat area:
https://www.wpc.ncep.noaa.gov/qpf/99epoints.txt